Bill's 02 Z-28 SS

Hi Folks,
The car is 97 Z-28 with a LT1/M6 with 120K miles. I was setting a code for the driver's side CAT, replaced it and now no codes. The car is running a little better now but still there is backfiring or breaking up at high RPMs (>3K). I have AutoTap, hooked it up and everything looks normal. I looked at the MISFIRE numbers - only cylinder 5 show a count of 2 and no history. However the car breaks up bad with no load above 3K and under normal driving conditions at 2K in 6th gear.
Is there something I should be looking at or checking out?? With no codes or bad readings, this has me stumped. I appreciate any help you can provide.
Thanks
Bill

WhyHelloOfficer

iTrader: (5)

It sounds ignition related.

Take some time and closely inspect all of your plug wires and plugs. Also a common problem is the connector going from the wiring harness to the opti is usually pretty worn out.

When I had some mid-range RPM misses it was caused by plug wires. Also take a look at the wire going from the opti to the ignition coil -- sometimes they aren't connected completely or the boot on the coil is just worn out.

Give us a little info and keep us updated.

Red97Z

iTrader: (22)

Mine had the same issue, problems above 3K

it's the opti.

Bill's 02 Z-28 SS

Hi "WhyHelloOfficer"
Everything you stated makes sense and more than likely is the problem. I inspected the wires and ohm'ed out - they checked ok including the coil wire. However, I ordered new wires and I am going to replace them based on your advice, why take a chance. Maybe under load they fall a part. I pulled the wiring harness going to the Opti unit, it looks good and bussed out OK also. I wish it was the problem.
One other thing it happened today, when the car was cold, I powered it up and rev'ed it up (which I never do to any of cars). It breakup real bad around 2500 and it would not get above 3000RPM.
thanks
Bill

Bill's 02 Z-28 SS

Hi Folks,
HAPPY NEW YEAR TO ALL!
I replaced plug wires and plugs, checkout the wiring(Ohm'ed them out) and I still have the problem above 3K. If I was setting a code, i would know what to do but not codes and the everything else looks normal. Anybody have another ideas?
thanks
Bill
PS: I found easy way to get PLUG #8 out, disconnect the battery, remove O2 senor next to the starter, and remove starter (remove the two bolts and slide the starter forward). you will have the room to work now.

WhyHelloOfficer

iTrader: (5)

Hey Bill,

Happy New Year to you too. With the plugs and wires being good -- I'd say you're going to have to look deeper into the ignition system. Because it is still behaving that way in open loop (when the car is cold) -- you're basically narrowed down to your ignition coil, ignition control module, and opti-spark.

Shoebox's Website (www.shbox.com) is a great LT1 resource, definitely check it out sometime.

Here is some stuff from his website to help you test more stuff before you go start throwing money at new parts.

Testing a signal is going from the Opti to the coil:
http://shbox.com/1/4th_gen_tech2.html#opti_test

Testing the Continuity of the coil:
http://shbox.com/1/coil_testing.jpg

Worst case scenario (replacing the Ignition Coil):
http://shbox.com/coil/coil.html

Bill's 02 Z-28 SS

HI Folks
Just to keep everybody posted, replaced the wired and plugs and the breaking up after 3k was corrected when warm. However, still have a couple of problems; when it cold, starts up right away and runs good. However, if I go above 3K when cold it breaks up - below that runs good. sit in the driveway with AutoTap connected and rev'ed it past 3K (Ikept the RPM above 3K to force something to happen and it did)- see a misfire#3, random Misfires, and Vehicle Speed Sensor. The Vehicle Speed Sensor does not make sense considering I was parked. First could the injector screwing up and dumping move fuel in that Cylinder?? Plug and wire are good - rechecked, any way it does not happen once the engine is run for a couple of minutes. It like it is flood, bogging down the engine, and then clears itself.
thanks

Bill

krayzie7th

does it drive normally like on the freeway and streets but when go WOT does just feel lack of power?my friend had a similar problem and we thought it was the opti but it wasnt, then we checked the vacum lines and they were set up wrong and that coused it to break up around 3k too.So you might want to check those out.

Bill's 02 Z-28 SS

Hi Folks
For the problem identified, the issue goes away. It is like it is being flooded or lack of gas for the first couple of minutes - PSI is about 41 lbs. On the open road I have different problem, it bucks under a little load but give it a little gas and it pulls well. I can put it in 6th gears at 40mph (1000 RPMs) and it works great, get over 1600 RPM and it is will buck - get above 2000 it works good.
thanks
Bill

ZEX-Z

iTrader: (4)

So what was the problem. my car is doing the same thing. new PCM,New optispark. New plugs, wires, MAP,TPS,CTS,MAP,O2's.What the hell. Need help. Badly. It still breaks up after 3K rpms and up when warm.

Bill's 02 Z-28 SS

Hi Zex-Z
I wish I knew - the car is still giving me problems. Once it warms up it is only at partial throttle position(<10%) does it act up and not all the time - greater than >10% is OK. I have noticed that on Colder days after warming up, the car runs good with minor issues - warmer days is when it starts acting up more. Totally confused on this one. I wish I could help both of us.
thanks
Bill

WhyHelloOfficer

iTrader: (5)

Could possibly be your ICM or Optispark -- basically when they're cold they are okay but once they start to warm up the extra heat causes them to not work properly.

Bill did you ever go to those pages on Shoe Box's website to test some of the different parts of the ignition system? If not I'd check that stuff out.

Testing a signal is going from the Opti to the coil:
http://shbox.com/1/4th_gen_tech2.html#opti_test

Testing the Continuity of the coil:
http://shbox.com/1/coil_testing.jpg

Bill's 02 Z-28 SS

Hi "WhyHelloOfficer"
I yes I did checkout the site - very good site and great information but did not help resolve the problem - everything checkout OK. MY car is real bad when first started - idles great - above 1.5K backfires. Drive a 1/4 mile or let it warm up in the driveway and it run OK - no more backfiring. Once it warmed up , a different problem happens between 1500 and 2000 RPMs (bucking)- below it or above - it works OK. I can put the car in 6th gear, 1000PRMs - it will pull great and NO issues at all. Cruise at 75 (6th gear) and I can feel the engine bucking - no all the time but it is a function of the outside temperature. Also throttle position – push a little on the throttle and OK - weird
Thanks
Bill